In the natural gas production and distribution industry, various methods of measuring physical properties of hydrocarbon gases have been developed. Properties whose values are important to the industry include pressure and composition. Other properties, such as calorific value, can be determined using these measurements.
One method of determining gas physical properties is gas chromatography. To use this method to determine, for example, calorific value, gas components are separated in a gas chromatograph. The composition of the gas is determined by comparing either peak heights or peak areas, or both, of the chromatograph, with reference components whose identities are known. The composition values are then used to calculate calorific values. A limitation of this method is that gas samples must be removed for testing. This limitation inhibits the use of chromatography for in-line measurements, such as when the gas is in a pipeline and samples must be obtained by re-directing a flow of gas from a tap in the line.
Gas combustibility, in particular, has been determined by various methods that require a sample of the gas to be separated out for testing. For example, one such method is to combust the gas and measure the heat released. Another such method is to combine the gas with a catalyst and measure the heat.
Other existing methods for determining gas properties use various spectroscopic techniques, which may be used for in-line analysis. One such technique is nuclear magnetic resonance (NMR) spectroscopy. The basic concept of NMR spectroscopy is the application of a magnetic bias field to a sample under test. The bias field comprises both a steady field and a radio frequency (RF) field. Nuclei of the sample absorb energy from the RF magnetic field and respond with an easily detectable signal when the ratio of the RF frequency to the strength of the steady field equals a resonance constant for the nuclei, i.e., the resonance response.
As an example of NMR measurements, it is known that hydrogen nuclei will selectively absorb and subsequently re-emit energy at 4.258 MHz in a 1000 Gauss steady magnetic field. At resonance, the amount of energy absorbed by the nuclei of a given sample from the RF field is directly proportional to the number of hydrogen nuclei in the sample. The energy absorbed and re-emitted is indicated by a signal, which is detected and used to obtain a quantitative measure of hydrogen content of the sample, i.e., the number of hydrogen nuclei in the sample. From this value and from other easily measured parameters, other properties, such as heating value, density, and compressibility can be derived.
One NMR method of gas analysis obtains NMR signal amplitudes from a high resolution frequency spectra, which represents the collection of signals emitted at slightly different frequencies by the selected nuclei in each molecular specie at resonance. This high resolution method requires a magnetic field with high homogeneity. To accomplish this test environment, it is often necessary to adjust the placement of the gas sample relative to the field. For example, some techniques spin the gas sample about the axis of the NMR sensor coil. Because of these requirements, high resolution NMR techniques are not suitable for in-line measurements.
Another method of using the magnetic resonance signal is transient NMR, in which the RF field is applied in the form of short bursts of energy. The emitted signal is analyzed on the basis of amplitude and relaxation time constants. The most basic NMR response is a transient signal from the absorbing nuclei that is referred to as the free induction decay (FID) signal. In a magnetic field of high homogeneity, the Fourier transform of the FID signal will produce the high resolution NMR spectrum signature. However, this method requires complicated signal processing.
Gas density, which is related to compressibility, is frequently determined by measuring the pressure and correcting for non-linear compressibility by use of a "z" factor to arrive at an accurate measure of the density. However, the "z" factor is dependent upon gas composition and must be measured regularly using relatively time consuming methods. The "z" factor is also highly dependent upon the gas pressure and small variations can introduce substantial errors.
In light of the limitations of the above techniques, there is a need for an improved in-line method of determining physical properties of a gas in a flow line. The method should permit properties such as calorific value and compressibility to be accurately determined.